Influenza A and respiratory syncytial virus ts mutants were evaluated in vitro and in experimental animals for evidence of genetic stability and antigenicity. A double lesion ts recombinant of influenza A virus was produced which contained 2 ts lesions, each of which was relatively stable as regards reversion to wild type properties. The further mutagenized ts-1 mutant of RS virus into which an additional lesion was induced with nitrosoguanidine was evaluated for genetic stability and was found to be significantly more stable than its ts-1 parent virus.